I have an aquarium and it has algae on the glass. The algae is somehow so attached to the glass that I´m afraid I'd scratch the glass if I tried to clean it using some hard plastic.
How can I remove the algae without damaging the glass?
If the algae is brown in color (diatom), has already settled and is hard; magnetic cleaners or algae-eating species wont be of much help. You can wrap a small piece of cloth around your hand and simply rub it off. You can also use an old toothbrush. Once the algae is off the glass, you can then use a magnetic cleaner or introduce algae-eating species as a maintenance/cleaning crew.
There are a few types of fish that feed on algae, and some are pretty cool looking.
I've found this list of algae-eaters, which is a great list because it lets you know whether your existing fish will find them tasty.
You can use a magnetic fish tank cleaner (one example).
It consists of two pieces that use magnets to attach through the glass. One piece is often shaped like a grip, the other piece has a surface used to clean off the algae.
You have to be careful the cleaning surface is free of sand or other dirt, otherwise you might scratch the glass. There are floating versions available which don't drop down to the bottom of the aquarium if the pieces lose contact. This reduces the risk of getting sand onto the surface and makes it easier to retrieve the piece that's in the water.